When does mocking make tests worse? What are the signs of over-mocking?

Mocks make tests worse when they replace the thing you should be testing, or when they tie the test to the implementation.

Signs of over-mocking:

• You set up five mocks to test two lines of code.
• Most assertions are toHaveBeenCalledWith on internal collaborators.
• A refactor keeps the behaviour identical, but dozens of tests break.
• A mocked repository returns data that the real query could never produce.
• You mock modules of your own code just to reach a branch.

The cost is double. Tests fail when nothing is broken. And tests pass when something is broken: a mocked repository can't catch a wrong WHERE clause or a constraint violation.

What to do instead:

• Mock only at architectural boundaries: the network, the message broker, the clock and randomness.
• Prefer fakes, or a real database running in a container.
• Assert on outcomes.

If a class needs many mocks, it often has too many responsibilities.